The present disclosure relates generally to digital current differential transmission systems and, more particularly, to a method for canceling transient errors in unsynchronized digital current differential transmission line protection systems.
In recent years, there has been an increased interest in the application of line differential relays to very long power transmission lines by utilities all over the world. This renewed attention to this technique is due, in large part, to the availability of digital differential relays operating over high-speed digital communication channels. The basic operating principle of current differential relaying is to calculate the difference between the currents entering and leaving a protected zone, such that a protection feature is engaged whenever the current difference exceeds a threshold level. Accordingly, line differential relaying requires that the information of the current flowing through each line terminal is made known to all other terminals.
Earlier line differential relays utilized analog communication channels, typically in the form of pilot wires in order to exchange the current values between terminals. The application of an analog line differential scheme over pilot wires was limited to a maximum distance of about 8-10 kilometers, due to several factors such as pilot wire capacitance resistance, extraneously induced voltages (e.g., ground potential rise), etc. On the other hand, the arrival of digital communications allowed relay manufacturers to encode the information exchanged between the relay terminals as logical zeros and ones. Thus, a preferred choice of communication media for digital differential relays has been a direct fiber optic connection, as it provides security and noise immunity.
One way to realize improved system sensitivity is through the use of synchronized sampling. Because differential current is computed as the total of the phasor currents measured at the same time at all terminals, the system sensitivity is thus ultimately limited by the accuracy of those phasor measurements. Accordingly, in order to eliminate such errors due to time misalignment, each phasor is constructed from a set of instantaneous current samples taken at precisely the same time at all terminals in the system. In a synchronized system (such as, for example, the L90 system offered by General Electric), this is achieved through the use of synchronized sampling clocks. Each terminal has a sampling clock that controls both the precise time when current samples are measured, as well as the assignment of phasor coefficients to each sample. The sampling clocks are synchronized to within a few microseconds of one other through exchanges of time stamps using one or more techniques for clock synchronization so as to prevent differential measurements from being contaminated by time misalignment errors.
However, not every existing current differential protection system implements the use of synchronized sampling. In these instances, as an alternative to completely replacing those systems with synchronized systems, it is still desirable to be able to improve upon the performance of an unsynchronized system.